Heavy impurities in pulp suspensions are generally separated with conventional screens by means of centrifugal force in separate scrap taps, while coarse fiber fractions and impurities are generally separated by a screen member whereby the pulp is divided into accept and reject portions.
Light impurities are more difficult to separate, especially at high pulp concentrations. The pulp concentration, however, is desired to be high, for example, from about 3 to 5%, in order to achieve a high production capacity and to avoid the transport of large liquid volumes in the screening system.
Light impurities, therefore, are usually separated in separate devices, such as hydrocyclones. This requires not only separate equipment, but also low concentrations, below about 1%, in order to achieve a good result. It is, therefore, a less attractive method to vortex clean the entire flow from a pulp line. Instead, it is possible to carry out pressure screening through very fine slits at the highest possible concentration, and thereafter to use a vortex cleaner for separating the light impurities from the reject flow from the screens.